CsB3 O 5  crystal and its nonlinear optical devices

ABSTRACT

The present invention relates to single crystals of CsB 3  O 5  having large dimension and high quality which can be grown by pulling methods. The single crystals of CsB 3  O 5  are useful as NLO materials. The NLO devices made of CsB 3  O 5  single crystals can be used in a laser system of high power density and relatively large divergence and posses a character of high SHG conversion efficiency. Moreover, the NLO devices of the present invention are capable of producing coherent harmonics of wavelength as short as 170 nm and tolerating larger processing error of crystals.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/310,471, filed Sep. 22, 1994, now abandoned, which is acontinuation of U.S. patent application Ser. No. 08/051,445, filed Apr.23, 1993, now U.S. Pat. No. 5,381,754 issued Jan. 17, 1995, claimingforeign priority from China patent application 92102773.7, filed Apr.23, 1992, now China Patent No. CN 1073729A, issued Dec. 24, 1994.

FIELD OF THE INVENTION

The present invention relates to CsB₃ O₅ single crystal, process formaking the same and nonlinear optical (NLO) devices made of singlecrystals of CsB₃ O₅.

BACKGROUND OF THE INVENTION

When a beam of laser propagates in a crystal with non-zero components ofthe second order polarizability tensor, the crystal will produce NLOeffects such as second harmonic generation (SHG), sum-frequencygeneration (SFG), difference-frequency generation (DFG) and parametricamplification (OPA). NLO devices such as second harmonic generators, upand down frequency converter and parametric oscillator can be preparedusing crystals having NLO properties (Please refer to U.S. Pat. Nos.3,262,058, 3,328,723, 3,679,907, 3,747,022, 3,949,323 and 4,826,283).Also, please refer to Dmitriev et al., section 2.8, pp. 22-24, Handbookof nonlinear Optical Crystals (Springer-Verlag, 1991), teaching crystalsymmetry and effective nonlinearities.

Second harmonic generation (SHG) is the most important NLO effect. Anelectromagnetic wave with a frequency of w propagating in a NLO crystalwill induce a polarization wave of a frequency of 2w. That is theso-called "SECOND HARMONIC GENERATION". The conversion efficiency of aSHG crystal is proportional to the effective SHG coefficient (d_(eff))square and the input laser power, and is also relative with thephase-matching condition. When other conditions are selected, if phasematching is achieved, the conversion efficiency will reach the maximum.Generally, there are two types of phase-matching: Type I wherein the twoincident waves have the same polarization; and Type II wherein the twowaves have orthogonal polarization. The most-commonly used method forachieving phase-matching is to find a suitable orientation of thecrystal as the propagating orientation of incident waves, and along thisorientation, the refractive indices are the same for both thefundamental and the second harmonic waves. When the orientation of thecrystal drifts from this special orientation, phase-mismatching willgenerally occur. The value of acceptance angle of a crystal reflects theaffecting extent on the conversion efficiency when the acceptance angledrifts from the phase-matching condition. In addition, due to theinfluence of the double refraction of crystal which results in thedifference between the energy propagation direction and the phasedirection, the fundamental wave and second harmonic wave will separateeach other after they propagate in the crystal for a certain distance.That is so-call WALK-OFF effect. The walk-off angle restricts the lengthof crystal having effective function.

Desirable NLO crystals should have the following requisites: greatnonlinear polarization coefficient; wide transparency range; goodphase-matching condition and high damage threshold; easy to grow; andeasy to obtain a single crystal with large dimension and high quality.

BBO (barium betaborate, low temperature modification: B-BaBhd 2O₄) andLBO (lithium triborate: LiB₃ O₅) are excellent NLO crystals of boratesdeveloped in recent years, and have been used widely in NLO devices,especially in NLO devices which can stand up to lasers with high powerdensity (See Scientia, Sinica, B28, 235, 1985; U.S. Pat. No. 4,826,283and Chinese Patent No. 88102084). It has been found that BBO has good UVtransparency ability (DV absorbing edge is 190 nm); high damagethreshold (15 GW/ cm², 0.1 ns, 1064 nm); and great effective SHGcoefficient (about 6 times of that of KDP). The main disadvantage of BBOis that the z component of its SHG coefficients is too small (d₃₁ <0.07d₁₁) to restrict its use in deep UV range and in laser systems of largerdivergence. In addition, because of acceptance angle of BBO (<1 mrad-cm)is too small, high working accuracy is needed for it.

The UV transparent ability of LBO (UV absorption edge is 160 nm) is thebest and the damage threshold of LBO SINGLE CRYSTAL (25 GW/cm², 0.1 ns,1064 nm) is the highest among the NLO crystals. However, LBO is anincongruent compound which must be prepared by flux method, the periodof its production is much longer (over 1 month), the yield is low, andthe cost for production is high.

J. Krogh-Moe first reported the crystal structure of cesium triborate,CsB₂ O₅ (Acta Crystallography, Vol. 13, 889-892, 1960; and ActaCrystallography, Vol. B30, 1178-1180, 1974). It crystallizes in thespace group P2₁ 2₁ 2₁, and is a biaxial crystal. The largest crystallinesize reported was only 0.10×0.17×0.46 mm³. A. J. Marlor et al studiedthe crystallization kinetics of CsB₃ O₅ from its undercooled melt usingmicroscope (Physics and Chemistry of Glasses, Vol. 16, 108-111, 1975).However, neither a single crystal of CsB₃ O₅ with a size large enoughfor examining its physical properties, nor devices made of CsB₃ O₅ haveever been reported until now. Further, CsB₃ O₅ is an anhydroustriborate, see A Comprehensive Treatise on Inorganic and TheoreticalChemistry, Vol. 5, Supplement 1, Part A, J. W. Mellor, Landolt-BornsteinNew Series III/7d2 Crystal Structure Data of Inorganic Compounds, pp 17,pp 85, Ed. Hellwege, Springer-Verlay, Berlin 1980; Christ, the AmericanMineralogist, 45, 334, 1960; Kocher, Bulletin Soc. Chim. France 3, 919,1968), references which detail the differences between anhydrous boratesand hydrated borates.

OBJECTIVE OF THE INVENTION

One objective of the present invention is to provide CsB₃ O₅ singlecrystals with enough size which can be used to prepare NLO devices andprocess for making the same.

Another objective of the present invention is to provide a NLO devicewhich can generate coherent radiations of wave-length as short as 170nm.

SUMMARY OF THE INVENTION

The present invention relates to a NLO device comprising means to directat least one incident beam of electromagnetic radiation into one crystalhaving NLO properties whereby electromagnetic radiation emerging fromsaid crystal contains at least one frequency that is different than thefrequency of any incident beam of radiation, wherein said crystal is asingle crystal of CSB₃ O₅.

By using the NLO devices made of CsB₃ O₅ single crystals of the presentinvention, and emerging radiation with a wave-length as short as 170 nmcan be generated; and they can work at power levels in excess of 10GW/cm², and no damage of the said device has taken place.

The single crystal of CsB₃ O₅ of the present invention is grown by theprocess comprising:

mixing a cesium salt with B₂ O₃ or H₃ BO₃ at such an amount to make themole ratio of Cs₂ O to B₂ O₃ being at 1:3;

heating and melting the mixture to obtain a melt of CsB₃ O₅, maintainingthe temperature of said melt at a temperature of about 50-200° C. higherthan the melting point of CsB₃ O₅ (about 837° C.) for about 5-20 hoursin a Pt crucible;

decreasing the temperature of the melt to a temperature of about 0-2.0°C. higher than the melting point of CsB₃ O₅ and inserting a seed of CsB₃O₅ crystal into the crucible from the top, keeping the seed in contactwith the melt, rotating the seed at a speed less than 45 rpm and pullingit upward at a speed in the range of 0-5 mm/hour;

maintaining the growth of crystal at the conditions mentioned above forabout 1-20 days;

cooling it to room temperature at a rate of 30-100° C. per hour.

According to the present invention, the said cesium salts include Cs₂CO₃, CsNO₃ and CsCl.

According to the present invention, the growth of CsB₃ O₅ singlecrystals is easier than those of LBO and BBO, the production period forthe growth of CsB₃ O₅ is shorter than those of LBO and BBO. Therefore,the cost for the production of CsB₃ O₅ is less than those of LBO, BBO.

BRIEF DESCRIPTION OF DRAWING OF THE INVENTION

FIG. 1 illustrates the principle of the typical NLO device made of asingle crystal of CsB₃ O₅ of the present invention.

FIG. 2 illustrates in block diagram the process flow of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail hereinafter.

CsB₃ O₅ can be prepared by the reaction of a cesium salt, for exampleCs₂ CO₃, CsNO₃, CsCl and the like, with B₂ O₃ or H₃ BO₃ according to anyone of the following chemical reaction equations:

(1) Cs₂ CO₃ +H₃ BO₃ →CsB₃ O₅ +H₂ O +CO₂

(2) Cs₂ CO₃ +B₂ O₃ →CsB₃ O₅ +CO₂

(3) CsNO₃ +H₃ BO₃ →CsB₃ O₅ +H₂ O +NO₂ +O₂

(4) CsNO₃ +B₂ O₃ →CsB₃ O₅ +NO₂ +O₂

(5) CsCl+H₃ BO₃ +O₂ →CsB₃ O₅ +H₂ O +Cl₂

(6) CsCl+B₂ O₃ +O₂ →CsB₃ O₅ +Cl₂

According to the present invention and as illustrated in FIG. 2, thesingle crystal of CsB₃ O₈ is prepared by an improved top-seeding method100 comprising:

(a) a mixing step 101 comprising combining a cesium salt from a source101a with B₂ O₃, or H₃ BO₃ from a source 101b at such an amount to makethe mole ratio of Cs2CO₃ to B₂ O₃ being at 1:3;

(b) a heating and melting step 102 of the mixture from step 101 toobtain the melt of CsB₃ O₅ in a Pt crucible, maintaining the temperatureof said melt at a temperature T1 of about 50-200° C. higher than themelting point of CsB₃ O₅ (about 837° C.) for about 5-20 hours;

(c) a step 103 of decreasing the temperature of the melt to atemperature T2 of about 0-2.0° ° C. higher ban the melting point of CsB₂O₅ and a step 104 of inserting a seed of CsB₃ O₅ crystal into thecrucible from the top, keeping the seed in contact with the melt, a step105 of rotating the seed at a speed less than 45 rpm including pullingit upward at a speed in the range of 0-5 mm/hour:

(d) maintaining the growth of crystal at the condition mentioned abovefor about 1-20 days; and

(e) a step 106 of cooling the hot grown crystal to room temperature at arate of 30-100° C. per hour and a step 107 of producing a grown crystalCsB₂ O₅, as indicated by numeral 3.

By adopting the above-mentioned method, a single crystal with a size ofdia. 20×35 mm³ (approx. 20×20×35 mm³) can be obtained. The inventor ofthe present invention have first discovered that the crystals of acompound having formula CsB₃ O₅ possess the following NLO properties;

1. A wide transparent range of wavelength:

The inventors of the present invention have examined the transmissioncharacteristics of CsB₃ O₅ single crystals, and have found that the CsB₃O₅ single crystals are transparent in the wavelength range of 170 nm to3000 nm.

2. Employing the method of prism minimum deviation, the inventors of thepresent invention also measured the principal refractive indices of CsB₃O₅. Using the least square fitting method, the Sellmeier equations wereobtained as follows: ##EQU1## 3. Large effective SHG coefficient:

The inventors of the present invention also measured the SHG powder dataof CsB₃ O₅ single crystals using powder SHG technique, and found thatthe effective SHG coefficient of CsB₃ O₅ is about 3 times of that ofKDP, similar to that of LBO.

4. Large z component of the SHG coefficients:

CsB₃ O₅ single crystals belong to the point group of D₂, and have oneindependent non-zero SHG coefficient d₁₄ which is the z component of theSHG coefficients. The value of the z component of the SHG coefficients(d₁₄) measured by the phase-matching method is about 0.468×d₁₁ (BBO).

5. High damage threshold:

The damage threshold of CsB₃ O₅ single crystals at different directionsis in the range of 20-28 GW/cm² for a 1053 nm, 1.0 ns laser pulse.

6. The phase-matching angle of CsB₃ O₅ single crystals is as follows:

=59.4° and =0°, for type I (SHG).

7. The acceptance angle of CsB₃ O₅ single crystals is 1.12 mrad-cm,larger than that of BBO (the acceptance angle of BBO is 0.6 mrad-cm).

8. The walk-off angle of CsB₃ O₅ single crystals is 1.76°, which issmaller than that of BBO (the walk-off angle of BBO is 3.2°).

It can be seen from above that single crystals of compounds having theformula of CsB₃ O₅ is a kind of novel NLO crystal having excellent NLOproperties. The NLO devices made of the CsB₃ O₅ crystals obtained by theprocess of the present invention have the major advantages of those NLOdevices made either of BBO or LBO crystals: high SHG conversionefficiency; low requirement for divergence; great resistance toradiation damage. In addition, they have the capability of generating UVradiations of wavelength as short as 170 nm.

FIG. 1 illustrates the principle of a typical NLO device made of asingle crystal of CsB₃ O₅. In the Figure, a coherent electromagneticbeam 2 produced by a laser 1 is introduced into a CsB₃ O₅ crystal 3. Theresultant emerging beam 4 is then caused to pass through a filter 5 sothat the beam of concern is obtained. In other words, the NLO devicemade of CsB₃ O₅ crystal of the present invention comprises means todirect at least one incident beam of electromagnetic radiation into theCsB₃ O₅ crystal whereby electromagnetic radiation emerging from saidcrystal contains at least one frequency different from the frequency ofany incident beam of radiation. The devices made of CsB₃ O₅ singlecrystal of present invention can be used as second harmonic generators,up and down frequency converters, optical parametric oscillators and thelike. For the SHG case, beam 2 is of fundamental frequency whiledeparting beam 4 additionally contains a wave of a frequencycorresponding with the first harmonic of beam 2, and the wave offundamental frequency is removed when the beam 4 passes through thefilter 5.

The said crystal 3 is so oriented that the crystallographic axes b and c(not shown) are at angles and respectively from the optical path throughthe crystal. Angles and are phase-matching angles of CsB₃ O₅ crystal 3.H. V. Hobden (J. Appl. Phys. 38, 4365, 1973) discusses the details ofphase-matching in a biaxial crystal.

EXAMPLE 1

A Pt. crucible with a size of 50 mm in diameter×40 mm in height wascharged by a homogeneous mixture of 79.2 g Cs₂ CO₃ and 89.6 g H₃ BO₃,and then placed in a crystal growth furnace. The furnace was then sealedwith a cover made of thermal insulation material which had a holedisposed on the center of said cover for the free entrance of seed. Thefurnace was heated rapidly to a temperature of 1000° C., and held atthis temperature for 10 hours and then cooled to 838° C. A seed crystalof CsB₃ O₅, cut along axis c and tied to a shaft with a Pt. wire, wasinserted into the melt. The seed was rotated at a rate of 20 rpm andpulled upward at a rate of 0.5 mm/hour. When the growth of the singlecrystal was near the end, the speed of pulling the seed crystal wasincreased to make it just out of the surface of the melt, and thencooled to room temperature at a rate of 60° C./hour. The final productwas a transparent single crystal of CsB₃ O₅ of size diameter 20×20 mm³(approx. 20×20×20 mm³). The period for the crystal growth was 2 days andthe period for the crystal production was 4 days.

EXAMPLE 2

A Pt. crucible with a size of 50 mm in diameter×55 mm in height wascharged by a homogeneous mixture of 118.2 g Cs₂ CO₃ and 133.8 g H₃ BO₃,and then placed in a crystal growth furnace. The furnace was then sealedwith a cover made of thermal insulation material which had a holedisposed on the center of said cover for the free entrance of seed. Thefurnace was heated rapidly to a temperature of 950° C., and held at thistemperature for 12 hours and then cooled to 837° C. A seed crystal ofCsB₃ O₅ cut along axis c and tied to a shaft with sa Pt. wire wasinserted into the melt. The melt was then cooled at a rate of 0.2°C./day and the seed was rotated at a rate of 30 rpm. After 14 days thegrowth was ended and the obtained crystal was pulled just out of meltand cooled to room temperature at a rate of 50° C./hour. The finalproduct was a transparent single crystal of CsB₃ O₅ of size up to21×25×20 mm³.

EXAMPLE 3

A Pt. crucible with a size of 45 mm in diameter×50 mm in height wascharged by a homogeneous mixture of 88.2 g CsNO₃ and 84.0 g H₃ BO₃, andthen placed in a crystal growth furnace. The furnace was then sealedwith a cover made of thermal insulation material which had a holedisposed on the center of said cover for the free entrance of seed. Thefurnace was heated rapidly to a temperature of 900° C., and held at thistemperature for 12 hours and then cooled to 837° C. A seed crystal ofCsB₃ O, cut along axis c and tied to a shaft with a Pt. wire wasinserted into the melt. The melt was then cooled at a rate of 0.1°C./day and the seed was rotated at a rate of 20 rpm with clock andanticlockwise direction alternatively. After 20 days the growth wasended and the obtained crystal was pulled just out of melt and cooled toroom temperature at a rate of 60° C./hour. The final product was atransparent single crystal of CsB₃ O₅ of size up to 27×17×14 mm³.

EXAMPLE 4

The same procedure of Example 2 was carried out by using 56.4 g CsCl and62.4 g H₃ BO₅ as raw material. The product obtained thereby was atransparent crystal of CsB₃ O₅ with the size of 12×10×5 mm³.

EXAMPLE 5

A Pt. crucible with size of 50 mm in diameter×55 mm in height wascharged by a homogeneous mixture of 125.9 g Cs₂ CO₃ and 81.1 g B₂ O₃,and then placed in a crystal growth furnace. The furnace was then sealedwith a cover made of thermal insulation material which had a holedisposed on the center of said cover for the free entrance of seed. Thefurnace was heated rapidly to a temperature of 950° C., and held at thistemperature for 14 hours and then cooled to 838° C. A seed crystal ofCsB₃ O₅ tied to a shaft with a Pt. wire was inserted into the melt. Theseed was rotated at a rate of 45 rpm and pulled upward at a rate of 1mm/hour. When the growth of single crystal was near the end, the speedof pulling the seed crystal was increased to make it just out of thesurface of the melt, and then cooled to room temperature at a rate of40° C./hour. The final product was a transparent single crystal of CsB₃O₅ with a size up to diameter 20×35 mm³ (approx. 20×20×35 mm³). Theperiod for the crystal growth was 2 days, and the period for the crystalproduction was 4 days.

EXAMPLE 6

A crystal obtained by using the procedure of Example 1 was cut into abody of 6×6×6 mm³ in size with =59.4° and =0° after determination of thecrystallographic axes a, b, and c, it was then placed in the opticalpath shown in the figure. The light source was a Q-modulation Nd: YAGlaser of wavelength =1064 nm. The emerging beam of radiation ofwavelength =532 nm was obtained.

We claim:
 1. A NLO device comprising means to direct at least oneincident beam of electromagnetic radiation into one anhydrous triboratecrystal belonging to the space point group P2₁ 2₁ 2₁ and having NLOproperties whereby electromagnetic radiation emerging from said crystalcontains at least one frequency different from the frequency of anyincident beam of radiation, wherein said crystal is a single crystal ofCsB₃ O₅ having an effective SHG coefficient that facilitates generatingsaid emerging radiation having a wavelength of at least 170 nm, saidsingle crystal of CsB₃ O₅ comprises a grown crystaline structurecomprising a seed crystal of CsB₃ O₅ and a crystaline compoundconsisting of a mixture of a cesium salt with B₂ O₃ in an amount to makethe mole ratio of Cs₂ O to B₂ O₃ be 1:3.
 2. A NLO device of claim 1,wherein said NLO device can work at power levels in excess of 10 GW/cm².3. A NLO device of claim 1, wherein said cesium salt being selected fromthe group of cesium salts consisting of Cs₂ CO₃, CsNO₃ and CsCl.
 4. ANLO device comprising means to direct at least one incident beam ofelectromagnetic radiation into one anhydrous triborate crystal belongingto the space point group P2₁ 2₁ 2₁ and having NLO properties wherebyelectromagnetic radiation emerging from said crystal contains at leastone frequency different from the frequency of any incident beam ofradiation, wherein said crystal is a single crystal of CsB₃ O₅ having aneffective SHG coefficient that facilitates generating said emergingradiation having a wavelength of at least 170 nm, said single crystal ofCsB₃ O₅ comprises a grown crystaline structure comprising a seed crystalof CsB₃ O₅ and a crystaline compound consisting of a mixture of a cesiumsalt with H₃ BO₃ in an amount to make the mole ratio of Cs₂ O to B₂ O₃be 1:3.
 5. A NLO device of claim 4, wherein said cesium salt beingselected from the group of cesium salts consisting of Cs₂ CO₃, CsNO₃ andCsCl.
 6. A NLO device of claim 4, wherein said NLO device can work atpower levels in excess of 10 GW/cm².